Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen in the United States with an incidence of 4.5 million cases annually. The objectives of this project are to assess the frequency of C. trachomatis infections in selected populations, to develop and evaluate sensitive nucleic amplification assays for diagnosis, and to study the immunopathogenesis of C. trachomatis and C. pneumoniae infections. Using three new molecular amplification assays (polymerase chain reaction [PCR], ligase chain reaction [LCR], and transcription-mediated amplification [TMA]) we screened over 2,000 male and 20,000 female patients in Baltimore. Over a 3-year period, the prevalence of chlamydia declined from 12.7% to 9.2% in STD clinics, remained level at 7.4% in family planning clinics and was elevated at 14% in students attending high school-based clinics. Significant predictors of infection included age < 20, more than one or a new sexual partner, inconsistent condom use, mucopus, ectopy, and cervical friability. Utilizing LCR detection for C. trachomatis infection in urine samples, we further documented a prevalence of 8.8% in 8,278 female Army recruits. In a cost-effective analysis, we documented that universal screening with non-invasive urine-based DNA amplification and use of self-administered vaginal swabs for other STDs offers the most cost-effective strategy for frequent screening of STDs, particularly in adolescents. In order to determine the frequency of C. trachomatis transmission among sexual partners, we screened 494 people in sexual partnerships. PCR amplification was twice as sensitive in detecting infection compared to culture, and demonstrated that male-to-female and female-to-male transmission frequencies were equal at 68%. Internationally, in a community-based trial of mass STD treatment in the Rakai district of Uganda, we documented relatively high rates of chlamydia and gonorrhea in 15-24 year-olds. Ten months after mass antibiotic treatment, there was a 50% reduction in both chlamydia and gonococcal infection. This study demonstrates that the vast majority of STD infections are asymptomatic, and that mass antibiotic treatment is effective in lowering infection rates. Future studies will address the effectiveness of mass treatment in limiting HIV transmission. We recently demonstrated a correlation between C. pneumoniae and atherosclerotic heart disease with the isolation of C. pneumoniae from an explanted heart with severe coronary atherosclerosis. C. pneumoniae has also been demonstrated in nearly 50% of coronary and carotid atheromas by immunocytochemistry and/or PCR. In vitro studies have demonstrated that C. pneumoniae can infect and proliferate in coronary artery endothelial cells and aortic artery smooth muscle cells. Future studies will continue to examine the etiologic role and immuno- pathogenesis of C. pneumoniae in the development of atherosclerosis.